With the widespread adoption of DVD players, the utilization of multichannel (greater than two channels) audio playback systems in the home has become commonplace. In addition, multichannel audio systems are becoming more prevalent in the automobile and next generation satellite and terrestrial digital radio systems are eager to deliver multichannel content to a growing number of multichannel playback environments. In many cases, however, would-be providers of multichannel content face a dearth of such material. For example, most popular music still exists as two-channel stereophonic (“stereo”) tracks only. As such, there is a demand to “upmix” such “legacy” content that exists in either monophonic (“mono”) or stereo format into a multichannel format.
Prior art solutions exist for achieving this transformation. For example, Dolby Pro Logic II can take an original stereo recording and generate a multichannel upmix based on steering information derived from the stereo recording itself. “Dolby”, “Pro Logic”, and “Pro Logic II” are trademarks of Dolby Laboratories Licensing Corporation. In order to deliver such an upmix to a consumer, a content provider may apply an upmixing solution to the legacy content during production and then transmit the resulting multichannel signal to a consumer through some suitable multichannel delivery format such as Dolby Digital. “Dolby Digital” is a trademark of Dolby Laboratories Licensing Corporation. Alternatively, the unaltered legacy content may be delivered to a consumer who may then apply the upmixing process during playback. In the former case, the content provider has complete control over the manner in which the upmix is created, which, from the content provider's viewpoint, is desirable. In addition, processing constraints at the production side are generally far less than at the playback side and, therefore, the possibility of using more sophisticated upmixing techniques exists. However, upmixing at the production side has some drawbacks. First of all, transmission of a multichannel signal in comparison to a legacy signal is more expensive due to the increased number of audio channels. Also, if a consumer does not possess a multichannel playback system, the transmitted multichannel signal typically needs to be downmixed before playback. This downmixed signal, in general, is not identical to the original legacy content and may in many cases sound inferior to the original.
FIGS. 1 and 2 depict examples of prior art upmixing applied at the production and consumption ends, respectively, as just described. These examples assume that the original signal contains M=2 channels and that the upmixed signal contains N=6 channels. In the example of FIG. 1, upmixing is performed at the production end, whereas in FIG. 2, upmixing is performed at the consumption end. An upmixing as in FIG. 2, in which the upmixer receives only the audio signals upon which it is to perform an upmix is sometimes referred to as a “blind” upmix.
Referring to FIG. 1, in the Production portion 2 of an audio system, one or more audio signals constituting M-Channel Original Signals (in this and other figures herein, each audio signal may represent a channel, such as a left channel, a right channel, etc.) are applied to an upmix device or upmixing function (“Upmix”) 4 that produces an increased number of audio signals constituting N-Channel Upmix Signals. The Upmix Signals are applied to a formatter device or formatting function (“Format”) 6 that formats the N-Channel Upmix Signals into a form suitable for transmission or storage. The formatting may include data-compression encoding. The formatted signals are received by the Consumption portion 8 of the audio system in which a deformatting function or deformatter device (“Deformat”) 10 restores the formatted signals to the N-Channel Upmix Signals (or an approximation of them). As discussed above, in some cases a downmixer device or downmixing function (“Downmix”) 12 also downmixes the N-Channel Upmix signals to M-Channel Downmix Signals (or an approximation of them), where M<N.
Referring to FIG. 2, in the Production portion 14 of an audio system, one or more audio signals constituting M-Channel Original Signals are applied to a formatter device or formatting function (“Format”) 6 that formats them into a form suitable for transmission or storage (in this and other figures, the same reference numeral is used for devices and functions that are essentially the same in different figures). The formatting may include data-compression encoding. The formatted signals are received by the Consumption portion 16 of the audio system in which a deformatter function or deformatting device (“Deformat”) 10 restores the formatted signals to the M-Channel Original Signals (or an approximation of them). The M-Channel Original Signals may be provided as an output and they are also applied to an upmixer function or upmixing device (“Upmix”) 18 that upmixes the M-Channel Original Signals to produce N-Channel Upmix Signals.